LIGHT
Reflection of light
• When light is reflected off of a mirror it forms an image.
Normal
Incident ray
Reflected
ray
Mirror
Reflection of light
• The law of reflection states that:
the angle of incidence = the angle of reflection
i=r
Normal
Incident ray
Reflected
ray
i
r
Mirror
Properties of reflection
• An image is formed when light is reflected.
• The image is the same size as the object and the same
distance behind the mirror as the object is in front.
Properties of reflection
• The image is laterally inverted (left is switched with right)
• The image is virtual (it’s not real)
Summary
• The normal at a point on a mirror is perpendicular to the
mirror.
• The image of an object in a plane mirror is virtual, the
same size, laterally inverted, and the same distance from
the mirror.
• For a light ray reflected by a mirror:
the angle of incidence = the angle of reflection
• Homework: pg. 117 #1,2
REFRACTION OF LIGHT
Refraction of light
• Refraction occurs when light pass from one transparent
medium (substance) to another. For example it happens
when light passes from air to glass.
Air
Glass
Changes
• Refraction occurs because the speed of the light changes
in different mediums.
• When it travels from a fast medium to a slow medium, the
light bends towards the normal.
Air
Glass
Changes
• When light travels from a slow medium to a fast medium,
the light bends away from the normal.
Glass
Air
Apparent Depth
• Why do things look different when they are under water?
Apparent Depth
• Because light waves are refracted when they change
medium, they make object appear differently. They create
an image of an object that is not exactly where the object
is.
Astronomical Refraction
• Light traveling from stars is refracted when it crosses into
the Earth’s atmosphere. A vacuum is a fast medium and
air is the slow medium, so the light bends towards the
normal.
Summary
• Refraction is the change of direction of light when it
travels from one transparent medium to another.
• Refraction is towards the normal when it goes from a fast
to slow medium.
• Refraction is away from the normal when it goes from a
slow to fast medium.
• Classwork: pg. 119 #1,2
REFRACTION INDEX
Refraction
• The change of direction of a light ray depends on which
substance to light is traveling in.
• Light well bend differently when traveling through water
than it would through glass.
The refractive index
• The refractive index of a substance is n.
n=
the speed of light in air
the speed of light in the substance
Example
• The speed of light is air is 300,000 km/s. The speed of
light in glass is 200,000 km/s. Calculate the refractive
index.
n=
the speed of light in air
the speed of light in the substance
300,000 km/s
n=
n=
200,000 km/s
1.50
The refractive index
• It is a measure of how much a substance changes the
direction of light.
• The refractive index for water is 1.33 and glass is 1.50.
Therefore, glass will change light more than water.
The refractive index
• How could we find the refractive index if we do not know
the speed? A man named Snell figured out how in 1618.
• Snell’s law:
sin i
n=
sin r
n – the refractive index
i – the angle of the incident ray from
the normal
r – the angle of the refracted ray from
the normal
The refractive index
• Remember that the angle of incidence and angle of
refraction are measured from the normal.
sin i
n=
i
Air
r
Glass
sin r
Example
• A light ray travels through air into water. The angle of
incidence is 50o and the angle of refraction is 35o.
Calculate the refractive index.
sin i
n=
sin r
sin 50
n=
n=
sin 35
1.34
Summary
• The refractive index of a substance is
n=
the speed of light in air
the speed of light in the substance
• For a light ray traveling from air into a transparent
substance
sin i
n=
• Homework: pg. 121 #1,2
sin r
TOTAL INTERNAL
REFLECTION
Refraction of Light
• When light crosses a boundary from a slow medium to a
fast medium, the light moves away from the normal.
Glass
Air
Refraction of Light
• The larger the angle of incidence the larger the refracted
angle.
Glass
Air
Refraction of Light
• There is an angle where the angle of refraction will be 90o.
• This is called the critical angle.
Glass
c
Air
Refraction of Light
• If the angle of incidence is larger than the critical angle,
there will be no refracted ray, only a reflected ray.
• This is called total internal reflection.
Glass
Air
Optical Fibres
• Optical fibres use total internal reflection to transfer
information. The light travels down the wire but does not
escape because it is reflected from the boundary.
Summary
• When a light ray travels from a slow medium into a fast
•
•
•
•
medium there is an angle where the refraction is 90o. This
is called the critical angle.
When the incident angle is less than the critical angle,
most of the light is refracted.
When the incident angle is at the critical angle the light is
refracted along the surface.
When the incident angle is greater than the critical angle,
all of the light is reflected. This is called total internal
reflection.
Homework pg. 123 #1,2
THE CONVERGING
LENS
Converging lens
• A converging lens changes the direction of light passing
through it so it is directed to one point called the focal
point.
• The following objects use converging lens:
Converging Lens
• The following is when rays travel parallel to the principal
axis of the lens.
Ray Diagrams
• When completing ray diagrams there are three important rays
to consider.
1. The ray traveling parallel to the principal axis is refracted
through the focal point.
2. The ray traveling through the center of the lens is not
refracted.
3. The ray traveling through the focal point is refracted parallel
to the principal axis
Applications of Lens
• The camera lens is used to make a smaller, inverted, real
image on a film.
• The distance between the lens and the sensor must be
changed depending on the distance of the object.
Applications of Lens
• A projector is used to give a larger, inverted, real image.
The Magnifying Glass
• The magnifying glass is used to create an image that is
virtual, upright and magnified.
Summary
• The principal focus of a converging lens is the point where
•
•
•
•
parallel rays directed straight at the lens converge.
The focal length of a converging lens is the distance from
the lens to the principal focus of the lens.
A real image is formed by a converging lens if the object
is further away than the principal focus of the lens.
Copy the table at the bottom of page 127 into your
notebook.
Homework pg. 125 #1,2 and pg. 127 #1,2
ELECTROMAGNETIC
WAVES
Review
• The index of refraction is:
n=
the speed of light in air
the speed of light in the substance
• The wave equation is:
v = fλ
Definition: Spectrum
• A spectrum is a way of comparing objects by ordering
them. The following is an “economic spectrum”:
Poor
Middle
Rich
• We can use spectra to compare things instead of using
charts or graphs.
The Light Spectrum
• Light from a lamp or the Sun is called white light. It
contains all of the colours on the visible spectrum.
• Light that is only one colour is called monochromatic.
• The following is a picture of all the colours:
“Roy G Biv”
Different colours
• Different colours have a different wavelength (λ) and a
different frequency (f).
• Red has a longer wavelength and a lower frequency.
• Violet has a shorter wavelength and a higher frequency.
Dispersion
• The different colours of light travel at different speeds in
glass.
• If white light is passed through a glass prism, the colours
will be refracted differently.
• This is called dispersion.
The Electromagnetic Spectrum
• The visible light spectrum (what we can see) is a small
part of a larger spectrum called the electromagnetic
spectrum.
• Electromagnetic waves (EM waves) are disturbances
that transfer energy from one place to another.
• The following are some examples of EM waves: radio
waves, microwaves, infra-red, ultraviolet, x-rays
The Electromagnetic Spectrum
Properties of EM Waves
• EM waves can have different frequencies and different
wavelengths.
• All EM waves have a speed of 300,000,000 m/s in a
vacuum.
Radio Waves
• Wavelength ≈ 1km
• Uses: communications (radio, TV, mobile phones)
Microwaves
• Wavelength ≈ 1m
• Uses: communications (satellites), heating food
Infra-red Radiation
• Wavelength ≈ 1mm
• Uses: cookers and heaters, cameras, optical fibre, TV
remote controls.
Visible Light
• Wavelength ≈ 1μm (micrometer 0.000001m)
• Uses: optical instruments, photography, seeing
Ultraviolet Radiation
• Wavelength ≈ 1nm (nanometer 0.000000001m)
• Uses: UV sun tan lamps, security markers, sterilizing
equipment
X-rays and Gamma Radiation
• Wavelength ≈ 1pm (picometer 0.000000000001m)
• Uses: X-radiography, destroying cancer cells
Problems
1) Which types of electromagnetic waves are transverse
and which are longitudinal?
All transverse.
2) How could x-rays give us images of bones?
They have short wavelengths so they are able to pass
through flesh. However, they are reflected by bones.
3) Which types of electromagnetic waves could harm
humans?
X-rays and gamma rays can kills cells or cause cancerous
cells. UV rays can damage your eyes or skin. Radio waves
and microwaves can have a heating effect on the body.
Problems
Summary
• Dispersion is the splitting of white light into the colours of
•
•
•
•
the spectrum. Light of one colour is called
monochromatic.
All electromagnetic waves travel at 300,000 km/s through
space.
The parts of the electromagnetic spectrum starting from
the largest wavelength are: radio waves, microwaves,
infra-red radiation, light, ultraviolet radiation, X-rays,
gamma rays.
Glue electromagnetic spectrum in your notebook.
Classwork – pg. 129 #1,2 and pg. 131 #1,2